Semiconductor manufacturers are currently using deep ultraviolet (DUV) lithography tools based on KrF-excimer laser systems, operating at wavelengths around 248 nm, as well as ArF-excimer laser systems, which operate at around 193 nm. Vacuum UV (VUV) tools are based on F2-laser systems operating at around 157 nm. These relatively short wavelengths are advantageous for photolithography applications because the critical dimension, which represents the smallest resolvable feature size that can be produced photolithographically, is proportional to the wavelength used to produce that feature. The use of smaller wavelengths can provide for the manufacture of smaller and faster microprocessors, as well as larger capacity DRAMs, in a smaller package. In addition to having smaller wavelengths, such lasers have a relatively high photon energy (i.e., 7.9 eV) which is readily absorbed by high band gap materials such as quartz, synthetic quartz (SiO2), Teflon (PTFE), and silicone, among others. This absorption leads to excimer and molecular fluorine lasers having even greater potential in a wide variety of materials processing applications. Excimer and molecular fluorine lasers having higher energy, stability, and efficiency are being developed as lithographic exposure tools for producing very small structures as chip manufacturing proceeds into the 0.18 micron regime and beyond. The desire for such submicron features comes with a price, however, as there is a need for improved processing equipment capable of consistently and reliably generating such features. Further, as excimer laser systems are the next generation to be used for micro-lithography applications, the demand of semiconductor manufacturers for powers of 40 W or more to support throughput requirements leads to further complexity and expense.
In laser systems used for photolithography applications, for example, it would be desirable to move toward higher repetition rates, increased energy stability and dose control, increased system uptime, narrower output emission bandwidths, improved wavelength and bandwidth accuracy, and improved compatibility with stepper/scanner imaging systems. It also would be desirable to provide lithography light sources that deliver high spectral purity and extreme power, but that also deliver a low cost chip production. Requirements of semiconductor manufacturers for higher power and tighter bandwidth can place excessive, and often competing, demands on current single-chamber-based light sources. Many of these obstacles are overcome by taking advantage of a dual-gas-discharge-chamber technology referred to herein as MOPA (Master Oscillator—Power Amplifier) technology. MOPA technology can be used to separate the bandwidth and power generators of a laser system, as well as to control each gas discharge chamber separately, such that both the required bandwidth and pulse energy parameters can be optimized. Using a master oscillator (MO), for example, an extremely tight spectrum can be generated for high-numerical-aperture lenses at low pulse energy. A power amplifier (PA), for example, can be used to intensify the light, in order to deliver the power levels necessary for the high throughput desired by the chip manufacturers. The MOPA concept can be used with any appropriate laser, such as KrF, ArF, and F2-based lasers.
Components of a MOPA laser system can include those discussed in U.S. Pat. No. 6,577,663, hereby incorporated herein by reference, which discloses a molecular fluorine (F2) laser system including a seed oscillator (or master oscillator) and power amplifier. The seed oscillator comprises a laser tube including multiple electrodes therein, which are connected to a discharge circuit. Seed radiation can alternatively be provided by an excimer lamp maintained at low pressure. The laser tube is part of an optical resonator for generating a laser beam including a first line of multiple characteristic emission lines around 157 nm. The laser tube can be filled with a gas mixture including molecular fluorine and a buffer gas. The gas mixture can be at a pressure below that which results in the generation of a laser emission, including the first line around 157 nm having a natural line width of less than 0.5 pm, without an additional line-narrowing optical component for narrowing the first line. The power amplifier increases the power of the beam emitted by the seed oscillator to a desired power for applications processing. A power amplifier (PA) typically includes a discharge chamber filled with a laser gas, such as a gas including molecular fluorine, and a buffer gas. Electrodes positioned in the discharge chamber are connected to a discharge circuit, such as an electrical delay circuit, for energizing the molecular fluorine in the chamber. The discharge of the PA can be timed to be at, or near, a maximum in discharge current when a pulse from the master oscillator (MO) reaches the amplifier discharge chamber. Various line-narrowing optics can be used, such as may include one or more tuned or tuneable etalons.
A major limitation to deep-UV (DUV) excimer laser systems is the lifetime of the optical components used therein. Especially at the level of output power on the order of tens of Watts, fast decay of optical components increases downtime as well as operational costs for applications such as microlithography. While implementation of the MOPA concept can increase the output power of the laser system, one of fundamental limitations of existing MOPA systems is this relatively short optics lifetime. For example, one of the principal factors limiting the optics lifetime in a MOPA system is the fast degradation of the output window of the amplifier discharge chamber, as this optical component is typically at the point where the power is at the highest level.